1. Field of the Invention
The present invention relates to a magnetoresistive head and, more particularly, to a magnetoresistive head used for reading information signals from a magnetic recording medium using a spin valve magnetoresistance effect.
2. Description of the Prior Art
As an electromagnetic transducer used for reading information signals from a magnetic recording medium such as a hard disk, a magnetic card, a magnetic tape or the like, a magnetoresistive sensor (MR sensor) having high read sensitivity has been known.
The electromagnetic transducer having a sectional structure shown in FIG. 1A has been constructed by using such phenomenon that electric resistance is changed according to differences in the direction of magnetization of magnetic layers and the direction of electric current. In general, this phenomenon is called as anisotropic magnetoresistance effect (referred to as AMR hereinafter). The AMR devices have been disclosed in Patent Application Publications (KOKAIs) 5-217123, 5-325138, 5-182147 etc., for example.
In FIG. 1A, on a substrate 1 made of a magnetic shield material, an insulating layer 2, a soft magnetic layer 3, a nonmagnetic metal layer 4, and an MR layer 5 are formed in that order. A pair of conductor lead layers 6a, 6b are connected on both sides of the MR layer 5 so as to put a sense region A therebetween. Antiferromagnetic layers 7a, 7b are formed between the conductor lead layers 6a, 6b and the MR layer 5. The MR layer 5 is magnetized from one conductor lead layer 6a toward the other conductor lead layer 6b by exchange interaction between the MR layer 5 and the antiferromagnetic layers 7a, 7b.
In such MR device, a magnetic field is generated around the MR layer 5 by flowing a sense current I from one conductor lead layer 6a toward the other conductor lead layer 6b, so that a biasing magnetic field which is perpendicular to the direction of initial magnetization M.sub.5 of the MR layer 5 is generated in the soft magnetic layer 3. The direction of magnetization M.sub.5 of the MR layer 5 is inclined by the bias magnetic field in the soft magnetic layer 3. Since the magnetization M.sub.5 of the MR layer 5 has a certain angle against the sense current I by the bias magnetic field, the electric resistance of the MR layer 5 has a linear response to an external magnetic field, as shown in FIG. 1B.
In the meanwhile, a nickel-iron (NiFe) film having a thickness of 200 .ANG. to 500 .ANG. has been known as a magnetic material constituting the MR layer 5. A MR ratio .DELTA..rho./.rho. thereof is not so large as 2 to 3%. In order to improve a reading effect, a material having a larger MR ratio has been desired.
Recently, as one method of attaining a higher MR effect, the electromagnetic transducer using a spin valve magnetoresistance effect have been proposed in Patent Application Publication (KOKAI) 4-358310.
The electromagnetic transducer has a structure shown in FIGS. 2A and 2B, for example.
In FIGS. 2A and 2B, a first ferromagnetic layer 12, a nonmagnetic metal layer 13, a second ferromagnetic layer 14, and an antiferromagnetic film 15 are formed in that order on a substrate 11. All layers from the first ferromagnetic layer 12 to the antiferromagnetic film 15 are patterned to have a rectangular plan shape. In addition, a pair of conductor lead layers 16a, 16b are formed at a distance in the longitudinal direction on antiferromagnetic film 15. Thereby, the magnetoresistance effect type transducer has been completed.
The first ferromagnetic layer 12 is made of a soft magnetic material such as NiFe. The direction of magnetization of the second ferromagnetic layer 14 is fixed by exchange coupling caused by the antiferromagnetic layer 15 connected to the surface of the second ferromagnetic layers 14. The direction of magnetization M.sub.b of the second ferromagnetic layer 14 is perpendicular to the surface of the magnetic recording medium (not Shown). The direction of magnetization M.sub.a of the first ferromagnetic layer 12 is directed along the surface of the magnetic recording medium. Thus directions of magnetizations M.sub.a and M.sub.b intersect with each other.
Since a magnetic field H of the magnetic recording medium is generated in the direction perpendicular to the surface of the medium, the direction of magnetization M.sub.a of the first ferromagnetic layer 12 is rotated according to the direction and magnitude of the magnetic field H of the magnetic recording medium, so that a relative angle between the directions of magnetizations of the first and second ferromagnetic layers 12 and 14 can be changed. When the direction of magnetization M.sub.a of the first ferromagnetic layer 12 is in parallel to the direction of magnetization M.sub.b of the second ferromagnetic layer 14, the resistance value becomes minimum. When the direction of magnetization M.sub.a of the first ferromagnetic layer 12 is in parallel and in the reverse direction to the direction of magnetization M.sub.b of the second ferromagnetic layer 14, the resistance value becomes maximum. Like the above, the resistance values of the first and second ferromagnetic layers 12 and 14 can be changed according to the change in strength of the magnetic field H generated by the magnetic recording medium, and then the electric resistance can be converted into the voltage so as to read information.
In case there exists no magnetic field H generated by the magnetic recording medium, a wide dynamic range can be utilized if the relative angle between the directions of magnetizations of the first and second ferromagnetic layers 12 and 14 is 90 degree, and thus it is preferable as a starting point of the operation.
However, in the electromagnetic transducer using the spin valve magnetoresistance effect, an iron-manganese (FeMn) alloy has been known as material of the antiferromagnetic layer 15 generating the above exchange coupling. But, since the FeMn alloy has poor corrosion resistance, it is oxidized upon fabricating the electromagnetic transducer, so that the characteristic of the transducer is deteriorated.
As another method of fixing the magnetization of the second ferromagnetic layer 14, such method can also be considered that a ferromagnetic layer (not shown) which has high saturation coercive force and high electric resistance is arranged near the second ferromagnetic layer 14 so as to achieve the exchange coupling therebetween.
The ferromagnetic medium having high saturation coercive force has good corrosion resistance. Therefore, the deterioration of the characteristic of the transducer not occurred. No detailed technology thereof have been disclosed in the above Patent Application Publication (KOKAI) 4-358310.
However, the magnetization M.sub.a of the first ferromagnetic layer 12 is intensively affected by the leakage magnetic field generated from the side section of the ferromagnetic medium layer having high saturation coercive force. As a result, there has been caused such problem that sensitivity to the magnetic field H from the magnetic recording medium is significantly deteriorated.